There are many reasons, but to get help you need to provide more
information, such as fuel type, what engine, what if anything led up to
the fault, is the engine running OK otherwise?
At some point, you are going to have to read the code from the ECU. You
will either need to buy a code reader, or try to find someone to do it
for you.
This group is for the Ford Focus; it gets very little traffic, so perhaps
you would do better to ask in uk.rec.cars.maintenance.
Chris

Most car parts stores will read the codes from the OBD computer in the
car for free and you can then google it for the most likely causes.
One simple thing that can cause it though is a loose fuel filler cap (it
hasnt been turned until it clicked). This causes excess fuel evaporation
that gets detected by the system and causes a code. If your fuel cap is
loose, tighten it and the light should go out after 20-30 min of driving.

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